Static electricity is a common problem. In industry, electrostatic discharge (ESD) events can be responsible for equipment failures, manufacturing defects and even explosions of solvents or flammable gases. One method of controlling static electricity is the use of static dissipative materials. Static dissipative materials are often required in manufacturing, the electronics industry and hospital environments. Examples of static dissipative materials include floorings in solvent handling areas and molded plastic trays for handling electronic components.
Static dissipative materials have electrical resistance between insulative and conductive materials. In general, materials that have a surface resistivity of more than 10.sup.12 ohms per square and/or a volume resistivity of more than 10.sup.11 ohm-cm are considered non-conductors, or insulators. Materials that have a surface resistivity of less than 10.sup.5 ohms per square and/or a volume resistivity of less than 10.sup.4 ohm-cm are considered conductive. Materials that have surface resistivities or volume resistivities in between these values are considered to be static dissipative. More specifically, static dissipative materials have surface resistivities between 10.sup.5 and 10.sup.12 ohms per square and/or volume resistivities between 10.sup.4 and 10.sup.11 ohm-cm. Some static dissipative applications require surface resistivity to be between 10.sup.6 and 10.sup.9 ohms per square and/or volume resistivity to be between 10.sup.5 and 10.sup.8 ohm-cm. (ESD Association Advisory for Electrostatic Discharge Terminology, ESD-ADV1.0-1994, published by the Electrostatic Discharge Association, Rome, N.Y. 13440.)
Surface resistivity is measured across the surface of a material. A typical method for measuring the surface resistivity of a material is to place electrodes on the surface, and then measure the resistance between the electrodes. The dimensions of the electrodes and distance between them is used to convert the resistance to surface resistivity in units of ohms per square.
Volume resistivity is measured through the bulk, or volume, of a material. A typical method for measuring the volume resistivity of a material is to place electrodes on the upper and lower surfaces of the material, and then measure the resistance between the electrodes. The area of the electrodes and thickness of the composite are used to convert the resistance to volume resistivity in units of ohm-cm.
Many commonly used materials are non-conductive. Examples of these are polymers, such as polyethylene or polysulfone, and epoxy resins, such as bisphenol A based resins. One method for making these materials static dissipative is to add conductive particles to them. Those non-conductive materials which are made static dissipative by adding conductive particles are called static dissipative composites. In order to make a non-conductive material static dissipative, conductive particles must be added in sufficient quantity to create a network of conductive paths through the material. These paths are formed by the conductive particles in electrical contact with each other. The level of conductivity depends on the number of conductive paths created by the particles. If there are too few particles, there will not be enough conductive paths to provide static dissipative properties to the composite.
Traditional conductive particles for static dissipative composites include carbon, graphite, and metal. These particles have several disadvantages. They are difficult to disperse and the static dissipative properties are strongly dependent on particle filling. This makes it difficult to produce composites within the desired conductivity range. These conductive particles are also dark in color and impart a dark color to the static dissipative composite.
JP SHO 53(1978) 9806 and SHO 53(1978) 9807 (Mizuhashi et al) teach glass microspheres with conductive indium oxide or tin oxide or indium tin oxide coatings. The object of JP SHO 53 (1978) 9806 is to produce glass microspheres with high conductivity without increasing the reflectivity of light too much. This reference teaches glass microspheres of transparent soda lime silicate glass, boron silicate glass, lead silicate glass, etc. with a low refractive index or high reflective index, or containing a coloring component. The manufacturing process includes a film formation process in which a solution containing a solvent, comprising water and/or a lower alcohol, a soluble-indium compound, and an organic thickener, is coated onto the surface of the glass microspheres to form a film on the surface of the glass microspheres. The next step is a drying process, in which the glass microspheres having a surface film formed from the above-mentioned solution are dried to evaporate the solvent in the film, and to form a film mainly composed of the above-mentioned indium compound and an organic thickener on the glass microspheres. This is followed by a baking process, in which the above-mentioned glass microspheres are baked in an oxidizing atmosphere at a high temperature to form a transparent coating mainly composed of indium oxide on the surface of the glass microspheres. A soluble tin compound can also be included with the soluble indium compound to form an indium tin oxide coating.
JP SHO 53 (1978) 9807 describes a method for making tin oxide coated microspheres that includes a solution production process in which an organic tin compound containing oxygen is dissolved in an organic solvent to produce a solution. The next step is a solution coating process in which the above-mentioned solution is coated onto the surface of glass microspheres to form a film on the surface of the glass microspheres. This is followed by a drying process in which the above-mentioned glass microspheres are dried under reduced pressure to form a resin-like film containing the organic tin compound on the surface of the glass microspheres. The final step is a baking process in which the above-mentioned glass microspheres are heated at a high temperature and under reduced pressure so that thermal decomposition of the organic tin compound is carried out to form a transparent tin oxide film on the surface of the glass microspheres.
Neither JP SHO 53(1978) 9806 nor JP SHO 53(1978) 9807 make reference to particles containing voids, such as hollow glass microspheres, nor do they disclose particles that have non-spherical shapes, such as glass fibers. These references also do not disclose the use of these particles for static dissipative composites. Both of these references declare that other methods for coating particles with diameters of 1 mm or less, such as sputtering, vacuum deposition, and chemical deposition are "difficult to apply," and state that "uniform formation of the film over the entire surface on the sphere is not possible," and "production equipment becomes expensive."
JP SHO 58(1983)-25363 (Tanaka) teaches pigment particles coated with indium oxide or tin oxide for conductivity. The particles are described as inorganic pigments. Inorganic pigment particles of the type listed in this reference are typically very small, on the order of several microns or less. This reference makes no reference to spherical particles, including those containing voids, such as hollow glass microspheres. Fibers other than asbestos are not taught. The objective of this invention is to provide a method of producing an inexpensive conductive pigment that can be used effectively as a recording material in electrophotographic or electrostatic recording systems or recording systems in which a color is formed by the passage of an electric current, and that can also be used to provide antistatic properties to polymer films, etc. This reference does not teach how to provide antistatic properties to polymer films, for example by describing the amount of conductive particles required for antistatic properties. The process for making these conductive pigments involves baking the pigments at a temperature between 400.degree. C. and 1000.degree. C. in the presence of indium or tin compounds.
U.S. Pat. No. 4,373,013 and U.S. Pat. No. 4,452,830 (both Yoshizumi) teach particles of titanium dioxide coated with antimony doped tin oxide (ATO). These inventions relate to ". . . a coated electroconductive powder suitable for use in applications such as forming electroconductive layers on paper for reproduction or duplication, such as electro-thermosensitive paper and electrostatic recording paper, and addition to resins to provide antistatic resins." The titanium dioxide particles of these patents preferably are "generally solid and have a specific surface area (BET method, N.sub.2 adsorption) in the range of 1 to 20 meter.sup.2 /gram (m.sup.2 /g) (corresponding to an average particle size of 0.07 to 1.4 micrometers) . . . ." The thickness of the ATO coating is preferably in the range of 0.001 to 0.07 micrometers (1 to 70 nanometers). The process for producing these electroconductive powders includes ". . . providing an aqueous dispersion of titanium oxide particles; providing a solution containing a hydrolyzable tin salt and a hydrolyzable antimony salt, said solution remaining free of hydrolyzed tin salt and hydrolyzed antimony salt; adding said solution to said dispersion under agitation while said dispersion is maintained at a temperature of 60.degree. to 100.degree. C. to hydrolyze said tin salt and said antimony salt as a result of contact between said solution and said dispersion, thereby to produce titanium oxide particles coated with antimony-containing tin oxide; and recovering the coated titanium oxide particles."
U.S. Pat. No. 4,568,609 (Sato et al) teaches a light permeable, electrically conductive material comprising a light permeable plate shaped particle, for example mica or glass flakes, with a conductive coating of "metal oxides doped with different kinds of metals." This material ". . . when compounded with transparent synthetic resin films or paints, is capable of providing a film or paint film with a superior conductivity without spoiling the transparency of said film or paint film." According to Sato, "[t]he plate substrate used for the present invention is itself required to be light permeable. The term used herein `light permeable substrate` or `light-transmittable plate substrate` implies such a plate substrate that when 2 wt % of the plate substance and 98 wt % of ethylene glycol are mixed, the resulting mixture is placed in a quartz cell having 1 mm of optical path length, and its transmittance is measured by means a hazemeter manufactured by SUGA Tester K.K. in Japan on the basis of the standard of ASTM D1003, the transmittance is evaluated to be 80% or more." Typically this measurement is referred to as "Total Luminous Transmission" or TLT. Therefore Sato requires that his core particles, which are plate shaped, have a TLT of greater than 80%.
This reference also teaches the use of these particles compounded into paints, plastics, or epoxies to form a light permeable, conductive film.
The process for making these coated particles includes preparing a plate substrate dispersion in an aqueous hydrochloric acid solution. A solution is made by dissolving tin and antimony chloride in concentrated hydrochloric acid, and this solution is dropped slowly in said mica dispersion and mixed. Metal hydroxides precipitate from the solution, coating the plate substrate. The coated plate substrates are washed and dried, then calcined at 350.degree. to 850.degree. C.
This reference states ". . . a spherical particle, even if conductive, has a narrow surface area as compared with different-shaped particles, and accordingly the probability of spherical particles contacting mutually is also low. Therefore, when intending to compound conductive spherical particles for instance with a film for providing said film with conductivity, it is impossible to render the film conductive satisfactorily without considerably increasing the amount of particles to be compounded." This patent makes no reference to fibers or hollow particles.
U.S. Pat. No. 5,071,676 and U.S. Pat. No. 5,296,168 (both Jacobson) teach ". . . an electroconductive powder composition comprising tens of microns to micron size particles having a surface coating layer of antimony-containing tin oxide which is conducting and an outer thin layer of a hydrous metal oxide having a thickness of from a partial molecular layer to 5 monomolecular layers, i.e., from about 5 to 30 angstroms, and an isoelectric point in the range of from about 5 to 9." Examples of the particles are titanium dioxide and amorphous silica. According to Jacobson, "[t]he hydrous metal oxide contemplated for use in the invention is an essentially non-conducting oxide . . . ." The isoelectric point represents the pH at which the surface of each particle has zero electrical charge, and, thereby, interactions of the individual particles with the resins of the paint system can controlled. These electroconductive powders are used as "pigments or additives in coating systems, such as for antistatic conductive paperboard." In addition, according to Jacobson, "[a]nother important use for electroconductive powders is as a component of the pigment in automotive paint primer compositions . . . ."
U.S. Pat. No. 5,104,583 (Richardson) teaches a "light colored conductive electrocoat paint" or "cathodic coatings." According to Richardson "[t]he electrically conductive pigment of the invention is a two-dimensional network of crystallites of antimony-containing tin oxide which exists in a unique association with amorphous silica or a silica-containing material. The antimony-containing tin oxide forms a two-dimensional network of densely packed crystallites on the surface of the silica or silica-containing material."
U.S. Pat. No. 5,284,705 (Cahill) teaches "an antistatic coating composition comprising a pigment portion dispersed in a fluent portion, the fluent portion containing a curable film-forming binder, the pigment portion containing tin oxide-rich electrically-conductive pigment, the proportion of said binder relative to the solids of said pigment portion being sufficiently high to provide a binder-continuous film when said composition is deposited and cured as a film on a substrate, the composition being characterized by an electrical conductivity-enhancing proportion of hard, impalpable achromatic filler mineral blended with said tin oxide-rich pigment."
U.S. Pat. No. 5,350,448 (Dietz et al.) teaches electrically conductive pigment particles. The coating that provides the conductivity is a halogen doped tin oxide and/or titanium oxide. These pigment particles optionally have a coating between the pigment particle and the conductive coating, which can be a metal oxide. This optional coating is provided for color or pearlescent appearance. The processes for making these include fluidized beds and wet chemical baths with tin or titanium chlorides and ammonium halides.
U.S. Pat. No. 5,376,307 (Hagiwara et al.) teaches a perfluorocarbon paint composition which has an "excellent anti-electrostatic property and release property." The composition is ". . . a fluorocarbon paint composition containing a fluorocarbon resin; and a hollow double-shell electroconductive material comprising hollow inner shells and outer shells coated on the surface of the inner shells and formed substantially of an electroconductive oxide; the ratio of the hollow double-shell electroconductive material in a coating component of the fluorocarbon paint composition being in the range of 1% to 30% by volume . . . ." This hollow double-shell electroconductive particle is then described as "having hollow inner shells formed substantially of amorphous silica or a silica-containing material, and outer shell formed substantially of tin (IV) oxide containing or doped with about 1% to 30%, preferably about 10%, by weight of antimony." According to Hagiwara, "[t]he paint according to this invention is suited not only for spray, brush or roll coating, but also for flow coating or immersion in applications where painting with relatively low viscosity is desired." In addition Hagiwara states, "[t]ypical applications of the fluorocarbon paint composition of the invention are for fusing rolls or fusing belts used in copying machines and printers, where the paint composition provides surfaces with both release property and anti-electrostatic characteristics . . . ." Additionally, "[t]he paint composition of the invention may be used to provide coatings surfaces of, for example, hoppers for transporting powder material, sizing rolls in paper manufacturing, feed rollers used in plastic film extruder, and textile sizing and drying rolls."
U.S. Pat. No. 5,398,153 (Clough) teaches fluorine and antimony doped tin oxide coatings on three dimensional substrates for use in static dissipative materials. Examples of these three dimensional substrates include "spheres, extrudates, flakes, single fibers, fiber rovings, chopped fibers, fiber mats, porous substrates, irregularly shaped particles, . . . ." Clough's process "comprises contacting the substrate with stannous chloride, in a vaporous form and/or in a liquid form, to form a stannous chloride-containing coating on the substrate; contacting the substrate with a fluorine component, i.e., a component containing free fluorine and/or combined fluorine (as in a compound), to form a fluorine component-containing coating on the substrate; and contacting the thus coated substrate with an oxidizing agent to form a fluorine doped tin oxide, preferably tin dioxide, coating on the substrate."
U.S. Pat. No. 5,476,613 (Jacobson) relates to an "electroconductive material comprising an intimate mixture of amorphous silica and a fine crystalline antimony-containing tin oxide, and to a process for preparing the same." According to Jacobson, "[t]he electroconductive powders of the invention when formulated with appropriate binders and additives may be applied to a variety of surfaces to impart electrical conductivity and antistatic properties . . . ." Additionally, "these ECP's are useful for coating glass, paper, corrugated boxboard, plastic film or in sheet such as polycarbonate, polyester and polyacrylate, electroconductive paint coatings, among many others." The term "ECP" as used in the reference refers to electroconductive powder.
U.S. Pat. No. 5,585,037 and U.S. Pat. No. 5,628,932 (both Linton) teach ". . . an electroconductive composition which comprises a two-dimensional network of crystallites of antimony-containing tin oxide which exists in a unique association with amorphous silica or a silica-containing material." One aspect of the invention is particles of amorphous silica that are coated with a two-dimensional network of antimony-containing-tin oxide crystallites. "The composition of this invention in a preferred embodiment comprises a powder which is particularly useful as a pigment in paint formulations for automotive paint systems. The finished powder of this invention comprises particles capable of forming a generally transparent conductive network with the paint film . . . ."
U.S. Pat. No. 5,631,311 (Bergmann et al.) teaches transparent static dissipative formulations for coatings. These electroconductive coatings "are comprised or consist of fine particles of an electroconductive powder, a thermoplastic or thermosetting resin, an organic solvent . . . ." According to Bergman, "[f]or the coatings of this invention to be transparent, the conductive powder is preferably comprised mostly of fine particles of size less than about 0.20 microns, that is smaller than half the wavelength of visible light." Also, "[t]he electroconductive coatings of the present invention are particularly useful in packaging materials which can be used, for example, to transport electronic parts."
U.S. Pat. No. 4,618,525 (Chamberlain et al) teaches metal coated hollow glass microspheres. This patent discloses tin oxide and aluminum oxide coatings as colorless coatings but does not provide examples of these coatings. This patent does not disclose tin oxide or aluminum oxide coatings as being conductive. This reference discloses a procedure for making coated particles by means of either sputter coating or vapor deposition, both of which are forms of physical vapor deposition (PVD).
U.S. Pat. No. 5,232,775 (Chamberlain et al) discloses particles with semiconductive metallic coatings for use in static dissipative polymeric composites. These coatings are preferably metal oxides, metal carbides and metal nitrides. Examples of the useful particles include ". . . particles fibers, milled fibers, mica and glass flakes, glass and polymeric microbubbles, talc and (subsequently coated) crushed microbubbles." The color of the coated particles or composites made from them is not disclosed. In fact, the coated particles and composites of the examples would all be expected to be brown to black in color. The coated particles of this reference are made by means of a sputter coating process.
U.S. Pat. No. 5,409,968 (Clatanoffet al) discloses metal coated particles for use in static dissipative polymeric composites. These particles are coated with a highly conductive metal followed by a coating of an insulating metal oxide. Examples of useful metals for the highly conductive metal layer include stainless steel and aluminum. An example of a useful insulating metal oxide layer is aluminum oxide. Examples of useful particles are glass, carbon, mica, clay polymers, and the like. The particles preferably have a high aspect ratio, such as fibers, flakes, rods, tubes and the like. The colors of these composites are not disclosed. The coated particles of this reference are made by means of a sputter coating process.
U.S. Pat. No. 4,612,242 (Vesley et al.); U.S. Pat. No. 5,245,151 (Chamberlain et al.); U.S. Pat. No. 5,254,824 (Chamberlain et al.); U.S. Pat. No. 5,294,763 (Chamberlain et al.); U.S. Pat. No. 5,389,434 (Chamberlain et al.); U.S. Pat. No. 5,446,270 (Chamberlain et al.); and U.S. Pat. No. 5,529,708 (Palmgren et al.) teach metal coated particles and metal oxide coated particles for various applications. These patents do not make reference to light colored coatings of conductive metal oxides.
Metal coated particles, as taught by U.S. Pat. Nos. 4,618,525, 5,232,775, and 5,409,968, and those of the paragraph above, such as glass microspheres or milled glass fibers that are coated with steel or aluminum can be dispersed easily into resins and polymers. They also have the advantage that once a minimum loading level is achieved the static dissipative properties of the composite are not strongly dependent on the filler concentration. This allows a better processing range for the filled material.
Another advantage of metal coated particles is the efficient use of metals. The core particle is effectively an extender of the metal. Metal coated particles can have the properties of metal particles, for example, conductivity, yet contain only a fraction of the metal. This is especially advantageous when expensive metals, such as indium, are used. In addition, metal coated particles are low in density when compared to solid metal particles. Metal coated hollow particles can have densities under 1 gm/cc. Even metal coatings on solid core particles, for example, steel coated glass fibers, can have densities less than 3 gm/cc, which is less than that of most metals.
Spherical particles have the additional advantage that they can be used at high volume loadings, without significantly increasing the viscosity of a resin. This allows the formulation of low viscosity, self-leveling composites for floorings and other coatings. This ability to use high volume loadings of spherical particles is also useful when volatile organic compounds (VOC's) need to be reduced in a composite formulation. Also, spherical particles do not line up when applied by such as a brush in a coating, or forced through an extruder die, such as when making a molded part. Fibers and flakes, on the other hand, do have a tendency to align when applied or extruded. This alignment can adversely affect the composite conductivity.
The metal coated particles are prepared by applying conductive coatings to the core particles using physical vapor deposition, in particular, sputter deposition. This physical vapor deposition process is surprisingly efficient and cost effective for producing coated particles. In addition, it is an environmentally clean process that does not involve solvents or liquid waste material. The coating material is almost entirely captured on the core particles. When using sputter deposition, the major source of waste is the metal left in the spent sputtering target. This metal is in a solid form that is easily reclaimable and recyclable. Alternative manufacturing processes, particularly wet chemical processes, involve disposal or recovery of contaminated liquids or solvents. There is often a great deal of metal in these liquids, which can be difficult to recover.
Metal coated particles do, however, impart color to composites. The coated particle color can vary from gray to black, or the coated particles can have a metallic color, such as copper, depending on the type of metal coating and the thickness of the coating. This has been a disadvantage in efforts to develop a market for metal coated particles for floorings and coatings in particular, especially when light colors are desired.